COOLING OF GAS INCLUDING IMPURITIES

FIELD OF THE INVENTION

The invention is related to an arrangement for keeping a plate heat exchanger clean in cooling of gas including impurities, e.g. in a cooling circulation of a process gas in cable production.

BACKGROUND OF THE INVENTION

When cooling gas including impurities by means of a plate heat exchanger, a part of the impurities is condensed in the heat exchanger which causes problems in cooling operation. The flow of the gas is hindered as the channels are gradually choked, and the cooling power is decreased also as impurities begin to accumulate on the surfaces of the plates.

Plate heat exchangers are primarily used e.g. for cooling process gas in vulcanization lines of cable production. Process gas to be cooled is conducted from the inlet of a heat exchanger into the slots between the cell plates and the cooled gas then via the outlet back to the vulcanization process. Vulcanization residue included in the gas is blocking particularly the openings through which the gas is conducted into the slots between the plates and is to some extent drifted also further on the surfaces of the plates. This causes deceleration of the gas flow and the cooling power which for their part cause changes in the process and quality problems. The changes start to build up immediately as the process is started.

Currently plate heat exchangers are normally cleaned by opening the exchanger and taking the plates separately for cleaning e.g. by means of a high-pressure cleaner. Repeated disassembly and assembly of the stack of plates cause gasket defects, and a heat exchanger must always be pressure tested after the assembly, which is laborious.

During the maintenance standstills cleaning may be made also by circulating washing detergent through the slots between the plates and flushing the exchanger finally with water. However, this kind of cleaning is not used very much.

When using these customary cleaning methods it is not possible to maintain continuous and smooth production process, but a production line must be shut down and time- consuming cleaning operations carried out always when it is necessary.

WO 2013085332 Al presents a solution in which a movable washing apparatus is placed in connection with the inlet channel of a plate heat exchanger, the washing apparatus taking periodically certain slots between the plates under washing treatment and closing the othes slots. In this way a powerful flow of washing agent is provided for the slots under cleaning, which makes the washing more efficient. Movable cleaning means in connection with the inlet channel are applied also in the applicant's solution, but otherwise the solution differs in many ways essentially from the solution of the cited publication. SUMMARY OF THE INVENTION

An object of the present invention is to provide a solution which makes possible to clean a plate heat exchanger during use and to which also various cleaning cycles executable during a stoppage of use without disassembly of the exhcanger may be combined.

In order to achieve the above object, an arrangement for keeping a plate heat exchanger clean in cooling of gas including impurities, wherein the plate heat exchanger comprises several cooling plates organized as a stack, in which the plates and the slots between them alternate, and an inlet channel formed by the openings made in the plates for conducting the gas to flow therethrough into the slots between the plates, and a corresponding outlet channel for conducting the gas out from the heat exchanger, is characterised in that it comprises:

in the inlet channel movable first means for forming a block between one or more slots between the plates and the inlet channel and opening a connection from said slots to a space having negative pressure in relation to the pressure in the heat exchanger and respectively closing said connection and opening said block for generating an instantaneous discharge of gas and cleaning blast from said one or more slots between the plates into said space with negative pressure, and

second means for controlling moving, forming of the block and opening the connection as well as opening the block and closing the connection for achieving said certain time taking gas discharge and cleaning blast function at each time from desired one or more slots between the plates.

In a preferred embodiment said first means comprise in the inlet channel a movable piston or equivalent equipment which extends to cover one or more slots between the plates and has at the both ends means for said blocking and in the middle an open hollow space and therefrom the openable and closable connection to said space with negative pressure.

In a further embodiment the means for moving the piston or equivalent equipment comprise a hollow shaft to which the moving means are coupled and which forms at the same time a part of the openable and closable connection to said space with negative pressure. In one embodiment the arrangement comprises means for bringing the gas to be cooled into the inlet channel from it's both ends for making possible that the gas is flowing into all other slots between the plates except the slots at each time blocked from the inlet channel.

Said one or more slots between the plates may include from 2 to 4 slots.

In an advantageous embodiment the arrangement comprises means for achieving said gas discharge and cleaning blast function one after the other in all the desired slots between the plates for executing a cleaning cycle covering extensively the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with reference to the accompanying drawings, in which:

Fig. 1 presents schematically and partially in cross-section an example of plate heat exchanger used in process gas cooling circulation in cable production;

Fig. 2 presents schematically and partially in cross-section the plate heat exchanger of Fig. 1 equipped with a possible arrangement according to the invention;

Fig. 3 - 5 illustrate the operation of the arrangement of Fig. 2;

Fig. 6 presents in further detail a possible structure and operation of the cleaning piston essential in the solution of Figs. 2 - 5; and

Figs. 7 - 10 present an example of the structure and operation of a precleaning apparatus designated to be placed before a heat exchanger in the cleaning circulation.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 presents schematically a typical plate heat exchanger 1 which may be used for cooling process gas Gl circulated in cable production. The heat exchanger is formed essentially by cell plates 3 stacked by means of gaskets 12 between frame plates 2a and 2b, so that the slots between the cell plates form a closed space into which the gas Gl is introduced via an inlet conduit 5, connection 4 and thereafter an inlet channel 6 formed by openings in the plates 3. Cooling water is flowing inside the cell plates. It is not necessary to consider water connections further here. Cooled gas G2 is conducted back to the process via corresponding outlet channel 8, connection 7 and conduit 9. The temperatures of the incoming and outgoing gas are monitored by means of corresponding temperature detectors 10 and 1 1.

Gas Gl is normally originally pure nitrogen used as a protection gas and pressure mediator in manufacturing cable insulation layers, typically e.g. in vulcanization of insulating material and a cooling phase after that. The pressure of the gas coming into a plate heat exchanger may be e.g. from 0.8 to 1.2 Mpa and the temperature from 50 to 70 °C, and it includes normally some amount of vulcanization residues the most of which are in gaseous state or in liquid state as microdroplets when coming into the heat exchanger. Due to cooling, the residues are condensed and sticked on the plate surfaces of the heat exchanger especially in the vicinity of the inlet connection 4. Accumulation of residues on the plate surfaces reduces quickly the gas flow and the power of the operation of the heat exchanger. As mentioned above when describing the background of the invention, the most usual procedure then is to stop the process, disassemble the plate stack of the heat exchanger, wash the plates one by one, reassemble the plate stack and the whole heat exchanger back to full performance, pressure test the exchanger and start the process again.

Figs. 2 - 6 present schematically an example of the basic solution according to the invention for arranging the cleaning of a plate heat exchanger during use. It comprises an arrangement in the upper part of the heat exchanger explained in detail with reference to Fig. 2. With reference to Figs. 3 - 5 the operation of this solution is explained in further detail, and the structure and operation of the essential part of this arrangement, a piston 17 provided with a tubular shaft 20, is explained with reference to Fig. 6.

The channel 6 in the upper part of the heat exchanger is brought through the heat exchanger, and the frame plate 2b is provided with a connection 13 similar to the connection 4 in the frame plate 2a. A conduit 15 equivalent to the inlet conduit 5 on the other side is coupled to the connection 13, and these conduits brings now together a bypass duct 14.

The structurally specific piston 17 sliding on the tubular shaft 20 is adapted by means of bearing and sealing 19 to an outwards directed connection 16 of the conduit 15 and is dimensioned according to the channel 6 so that it is on the one hand movable in the channel and on the other hand sealable in relation to the edges of the openings in the plates 3 forming the channel 6.

The mechanism sliding on the tubular shaft 20 and the bearing in the connection 16 is indicated by marking ME in Fig. 2. It comprises first a downwards directed gas and residue exhaust conduit 21 and there a controllable closing valve 22. The controllability is indicated by marking C2. The gas G3 cleaning and carrying residues out is exhausted under control via this conduit. At the end of the shaft 20 also a conduit 23 is placed for pressurised gas P, e.g. nitrogen, which is necessary for the operation of the piston 17. The conduit is provided with a controllable valve 24 the controllability being indicated by marking CI .

Furthermore, at the end of the tubular shaft 20 also a conduit 25 for hot water or vapor V is placed which is opened and closed by means of a valve 26 according to control C3. This equipment which is independent in relation to the solution of the invention is included in the heat exchanger for making possible washing by means of hot water or vapor, a possible embodiment of which is explained in further detail later. For moving the mechanism ME and piston 17 the arrangement comprises a linear actuator 27 the lead screw mechanism 28 of which moves the shaft 20 by means of an arm 29 attached to it. The control of the linear actuator may be realised in many ways. Here it is presented only schematically by a control unit 40. In Fig. 2 the piston 17 is in its extreme position against the connection 16 and sealing 19.

In the following, some examples of the operation of the arrangement acccording to the invention are explained in further detail with reference to Figs. 3 - 6. In Figs. 3 - 5 moving of the piston 17 is indicated by marking M. In Fig. 3 the piston is aligned with the connection 13 so that it closes totally the flow of gas through the connection. The plate heat exchanger operates then exactly similarly as without the arrangement of the invention. Gas Gl flows from the inlet connection 4 into the channel 6 and onwards into the slots between the plates 3. This is the preferred place of the piston between the cleaning cycles during use.

When cleaning during use, the piston 17 is moved step by step so that it in this case is always placed to cover two adjacent slots between the plates 3, first the first two slots, then the following two slots, and so on. In Figs. 4 and 6 the piston is placed to cover the slots S7 and S8. Incoming gas Gl can flow into all the other slots either directly from the connection 4 or via the bypass duct 14 and the connection 13. When step moving or equivalent control has moved the piston to the next place, pressure is connected via the conduit 23 into a tube 39 (Fig. 6) extending into the piston inside the tubular shaft 20 and onwards via channels in the piston under gaskets 38, which causes that the gaskets are pressed against the edges of the openings in the corresponding plates 3. As the pressure under the gaskets is removed, the pressure in the heat exchanger outside the gaskets release the sealing pressure and the piston may be moved to the next place.

In the piston 17, only the ends are sealed in the way described above. The middle part 37 of the piston is hollow and open to the tube formed by the shaft 20. As the piston is during the cleaning operations sealed to its place, the exhaust valve 22 is steered to open for e.g. some seconds. As the exhaust conduit 21 is connected into e.g. a tank with normal air pressure in which the impurities are gathered, a quick discharge of gas indicated by marking G3 is generated towards the direction contrary to the direction of normal gas flow. Suction caused by momentary negative pressure and a pulse-like gas discharge affects most powerfully just the area in which the most of the residues are sticked, and so the impurities are cleaned efficiently.

After the some seconds cleaning blast the valve 22 is closed as well as also the valve 24 disconnecting the sealing pressure, after which, as the gasket pressure is released, the piston 17 may be moved to the next place. The piston may be provided with valves or other equivalent means for releasing the gasket pressure more quickly so that the movement may be started more quickly. In the new place the above described cleaning function is carried out after which the piston is moved to the next place, and so on.

In Fig. 5 the piston 17 is finally moved to cover the last two slots between the plates 3. One end of the piston closes then the connection 4 and the flow of the gas Gl through it into the heat exchanger. Therefore the flow goes now during this cleaning function totally via the bypass duct 14. When this cleaning blast is carried out the piston is moved back to the position indicated in Fig. 3 from which the cleaning cycle was started and to which it is this time finnished, the cleaning arrangement staying now in stand-by state.

The operation of the arrangement according to the invention may be realised e.g. so that the cleaning cycles are started on grounds of certain criteria, e.g. slowdown of the flow of the gas or weakening of the cooling power, either automatically or manually. The cleaning cycle to be carried out may be like that one described above. It is possible also that the cycle is finnished by moving the piston still further to close only the inlet connection 4 and the next cleaning cycle is carried out by starting from this position and finnishing to the position of Fig. 3 from which the next cleaning cycle is started, and so on.

Cleaning cycles may be controlled also on grounds of the change of the heat transfer capacity, wherein the change of the temperature and the volume of water flowing in the heat exchanger are measured. From these the cooling effect is obtained, and the cooling cycles may be timed on grounds of the changes of the cooling effect.

Weakening of the gas cooling may be measured by means of the thermocouples 10 and

1 1 indicated in Fig. 2 by measuring the temperatures of incoming and outgoing gas. One possible way of controlling is to time the cleaning cycles according to experience and estimation capability obtained. Starting of the cleaning cycles may be steered also manually e.g. on grounds of the temperature measurements. Timing of the events in a cleaning cycle, durations, moving directions and distances etc. may be selectable e.g. by means of a menu presented by a control unit.

Because cleaning during use disturbs remarkably the flow of the cooling gas in a heat exchanger this must be taken into account when estimating the capacity of a heat exchanger needed. If the cleaning arrangement is carried out in heat exchangers already in use in production processes, it is therefore reasonable to increase the amount of plates in the exchangers. A suitable general advice may be to increase as many plates as the piston covers.

Fig. 2 presents also a way in which an as such known solution, i.e. cleaning the heat exchanger between production periods by circulating hot water or vapor through the exchanger, may be combined with the arrangement according to the invention utilizing to some extent same equipment. In this way cleaning under use may be improved so that disassembly of the heat exchanger may be avoided as long a possible. The conduit 25 provided with the controllable valve 26 mentioned earlier with reference to Fig. 2 is related to this improvement and makes possible to conduct hot water or vapor via the tubular shaft 20 and the piston into the heat exchanger.

The outlet channel 8 is opened also to a connection 30 in the side of the frame plate 2b for discharging water and vapor and on the other hand to a conduit 31 for discharging impurities. A pipe 35 provided for discharging vapor is provided with a controllable valve 36. For discharging impurities brought by water or vapor, the conduit 31 is provided with a branch conduit 32 equipped with a valve 33. For separating impurities from vapor, there is a perforated tube 34 inside the conduit 31 for vapor, and the most of the impurities remain outside the tube and are discharged via the conduit 32.

For carrying out a hot water or vapor washing, a heat exchanger is preferably provided with cooling water discharge means so that it is possible to raise the temperature of the cell plates high enough. Hot water or vapor is produced e.g. by means of a hot water cleaner in which the control keeps the water temperature below 130 °C. Low washing pressure is used so that no problems are caused to the plates and gaskets of the heat exchanger.

It is advantageous to let cautiously vapor flow through the plate cell before a cleaning cycle, so that differences in thermal expansion between plates do not damage plates or gaskets. However, hot water or vapor is conducted into the slots between the cell plates via the hollow shaft 20 of the piston, and the piston is moved in suitable intervals from one place to a next one, until the cleaning cycle is carried out, vapor flow closed and the piston moved back at the connection. In this way uniform cleaning is ensured for all slots between plates. The cleaning is advanced mainly like a cleaning under use but now only the valve 26 is opened as the piston is sealed in a new position.

In conclusion, condensed water and cleaning residue are so gathered to be discharged via the conduit 35.

Hot water or vapor washing may be carried out also so that cleaning blasts are directed contrary to those described above, and furthermore, the arrangement may be also such that it is possible to select the direction of the cleaning blasts.

For example, the inlet conduit and channel, related connections, bypass duct and the arrangement according to the invention for carrying out the gas blast cleaning cycles may be placed at a lower position in a plate heat exchanger and the outlet conduit, respectively, at an upper position. On the other hand, hot water or vapor washing equipment may then be arranged so that cleaning water or vapor is introduced into the heat exchanger via the outlet channel and related connections at the upper position and the discharging means for water or vapor as well as gathered impurities are placed suitably in lower inlet connections and conduits. A remarkable advantage of this alternative is that in hot water or vapor washing the direction of the cleaning medium is from the clean part to the contaminated part of the exchanger and then out.

For preventing cleaning residue to drift during hot water or vapor washing into gas circulation pipes, it is possible to provide as well inlet as outlet conduits with e.g. ball valves which are closed for the period of hot water or vapor washing.

For monitoring and evaluation of need or results of cleaning, a plate heat exchanger may be provided also with suitably placed conduits for inspections by means of an endoscope, which reduces further needs to use diadssembly for making sure adequate cleaning of the exchanger.

For minimizing needs to clean a heat exchanger it is advantageous to mount a preliminary filter in the icoming gas pipe immediately after a point in which the gas leaves a production line, whereby a remarkable portion of residues included in the gas may be removed there. A suitable solution for a preliminary filter is described with reference to Figs. 7 - 10.

A preliminary filter comprises a cylindrical shell 51 having a convergent conical lower part 52 around a tube 53 extending from the top of the shell through it in the lower end. The top of the shell 51 is provided with a collar 57 and a lid 58 attached to it closes there the shell as well as the tube 53. The lid is provided with openable and closable conduits 59 to which an endoscope may be coupled checking if the filter needs cleaning. The upper part of the cylinder is surrounded by a mantle 60 forming there a space for cooling water, the mantle being provided with an inlet valve 61 and an outlet valve 65.

Impurities are actually cleaned by circularly curved perforated plate structures 62 which extend between the cylinder part of the shell 51 and the tube 53 from the lid to a floor plate 54 between the cylinder part and conical part of the shell. The plate structures include each an open segment and are placed in relation to each other so that they form a kind of labyrinth in the way illustrated in Fig. 8.

Gas Gl is conducted into the filter via a conduit 64 placed close to the lower end of the cylinder part and flows through the labyrinth formed by the perforated plate structures 62 and further through an opening 3 close to the lid into the tube 53 through which it goes out from the filter.

Figs. 9 and 10 illustrate in further detail a possible plate structure which comprises in the middle a normal homogeneous plate 66B and at the both sides of it equivalent perforated plates 66C and 66A kept apart from the middle plate by rods 67. The labyrinth layout and the plate structures with perforation 69 in outer plates described above form for gas a lengthened route along the perforated surfaces whereby residue drops formed on the holes and surfaces run downwards on the plate surfaces and finally through the perforated floor plate 54 into the conical space 55 from which the residues are at certain times drained out via a valve 56. Heavy impurity elements remain in the filter also for gravity as the inlet conduit is in the lower part of the filter and the gas is going out from the labyrinth on top of the central tube 53.

The lid 58 of the filter shell may be opened and the curved plate structures 62 raise out for washing by vapor or high-pressure water when residues gathered on the plate surfaces disturb too much the operation of the preliminary filter.

It is obvious that there are a lot of alternative ways to accomplish a preliminary filter so that a remarkable portion of the impurities are removed from gas to be processed further.

Cleaning arrangement according to the invention makes possible to run long production periods without breaks as the gas cooling circulation is kept steady without disruptions. A further advantage of the repeated cleaning cycles during use is that vulcanization residues are then still in liquid form and cleaning blasts contrary to gas flow direction are able to remove the residues from the surfaces. If the residue is allowed to stay on the surfaces, it is dried and sticked there fast and then blocks bit by bit the flow paths.

The invention may vary within the scope of the accompanying claims.